In a hot gel, how does the flexing of metal plate cause the chemical reaction?

"A heat pack like the one you are describing contains sodium acetate and water. It turns out that sodium acetate is very good at supercooling. It "freezes" at 130 degrees F (54 degrees C), but it is happy to exist as a liquid at a much lower temperature and is extremely stable. Clicking the disk, however, has the ability to force a few molecules to flip to the solid state, and the rest of the liquid then rushes to solidify as well. The temperature of the solidifying liquid jumps up to 130 degrees F in the process.

When you boil the solid, you melt it back to the liquid state. You have to completely melt every crystal, by the way, or the liquid will quickly re-solidify. You can repeat this cycle forever, theoretically, just as you can freeze and melt water as many times as you like. The plastic pouch eventually wears out and leaks, though (since sodium acetate is a food additive, it is non-toxic)."